High temperature superconductors of the perovskite structure were originally produced from mixed oxides or oxide precursors. They have more recently been produced by very complex processes involving high vacuum, electron beams, and chemical vapor deposition. Most conductors prepared by such processes are limited to small or short dimensions, whereas electroplating is easily amenable to continuous production of long wires and ribbons.
The most common high temperature superconductive perovskite is YBa.sub.2 Cu.sub.3 O.sub.7, wherein the three metallic elements present are of very different electrochemical characteristics. The amount of oxygen present in this molecule is known to be slightly less than 7, but numerals are used here for brevity. In relation to hydrogen, the electropotentials of these three metals are:
______________________________________ Copper +0.34 volt Barium -2.91 volts Yttrium -2.37 volts ______________________________________
Where elements differ in electropotential as these metals do, they are extremely difficult to codeposit in a defined ratio. Where elements are highly electronegative in relation to hydrogen, as are barium and yttrium, they do not normally electroplate from an aqueous solution. Thus concurrent electroplating to make a deposit with elemental ratios 1:2:3 for YBa.sub.2 Cu.sub.3 O.sub.7 is extremely difficult to accomplish. The ratio of deposition may be affected by the solution concentration, pH, the voltage difference between anode and cathode, temperature, and presence of chemicals that might form complexes.
There has not heretofore been provided an efficient and effective process for concurrent electroplating of yttrium, barium and copper from aqueous solution. Whereas other electrodeposition processes have been researched and patented, the uniqueness of the present invention is evident in the discussion that follows.
The term electrodeposition covers placement of a material using an electromotive charge. When this is done from a liquid matrix, deposition can take place in one of two distinct processes:
1. Electroplating, wherein cations (+ charge) of a specific metal are attracted to the cathode (- electrode) and the ions are reduced to the elemental (metallic) state by addition of electrons to ions. The metal plating grows an atom at a time and generally forms a coherent metallic deposit.
2. Electrophoresis, wherein solid particles (not dissolved ions) are given an electrostatic charge (+ or -) and are attracted to an electrode of opposite charge, where the particles are physically agglomerated onto the substrate. Electrophoresis can be accomplished from colloids or dispersions and can sometimes be included with an electroplating process as noted below. Sometimes charged particles are deposited and agglomerated; sometimes charged colloids are deposited, as in latex electrodeposition; sometimes charged colloids containing mineral pigments are deposited, as in electrodeposition of automotive primers. See Sax & Lewis, Hawley's Condensed Chemical Dictionary, 11 ed, 1987, pp 457-458, Van Nostrand Reinhold Co, New York, incorporated herein by reference.
Electroplating can be accomplished from three different forms:
1. Single or mixed molten salt (e.g., aluminum manufacturing).
2. Aqueous solution (e.g., conventional copper, nickel and silver plating).
3. Nonaqueous solution (e.g., as described in U.S. Pat. No. 4,975,417).
Electroless or autocatalytic plating of metals is another form of deposition, but it is not electrodeposition and is distinct from the processes discussed herein.
The patents of Maxfield (U.S. Pat. No. 4,939,308) and Koura (U.S. Pat. No. 4,975,417) relate to concurrent electrodeposition of yttrium, barium, and copper. The Maxfield patent describes electroplating; the Koura patent describes electrophoretic deposition.
Barium, yttrium and copper have been concurrently electroplated from nonaqueous solution, as reported by Maxfield in U.S. Pat. No. 4,939,308. Maxfield used nonaqueous solvent (dimethyl sulfoxide) because barium and yttrium can not normally be plated from aqueous solution. Maxfield specifically addressed this problem in column 2, lines 62-65 and column 3, lines 4-7. Maxfield allowed water content not exceeding that which would be coordinated with the dissolved ions (water of hydration), which is calculated to be 0.2% to 1.6% water for the examples given by Maxfield. By contrast, the process of the present invention involves electroplating in which water is the principal solvent.
Publications describing a process similar to that of Maxfield are: Bhattachary, et al, "YBaCuO Superconductor Thin Film via an Electrodeposition Process", J. Electrochem Soc vol 136, no. 6, June 1991, pp 1643-1645 and Minoura et al, "Preparation of YBa.sub.2 Cu.sub.3 O.sub.7 and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.7 Films by Electrodeposition Techniques", Chemistry Letters, 1991, pp 379-382.
U.S. Pat. No. 4,975,417 (Koura) describes electrophoretic deposition of particulate barium carbonate from suspension onto a conductive substrate. This is not metallic electroplating as in the case of the Maxfield patent. The liquid media for this electrophoresis involves a mixture of organic solvents (e.g., isopropanol and acetone, methyl isobutyl ketone and ethyl ether) and the metal salts are not soluble in these solvents. Charging materials such as iodine and tetramethylammonium hydroxide are used, and electropotentials range from 50 to 500 volts in order to form a good agglomeration of the deposited particles. Koura used mixtures of barium, yttrium and copper carbonates or oxides in the liquid medium and the deposit was in the form of mixed oxides. Subsequently, those oxides were fused together to form a perovskite superconductor. The Koura patent is clearly stated to be an electrophoretic process and the immediate product is an agglomerate, distinctive from electro-plating. Distinctions of the Koura process include the high voltages and the use of dispersions rather than use of solutions.
Iwata et al (U.S. Pat. No. 4,939,119), describe superconductive elements which are electrodeposited from a dispersion or suspension onto a substrate. Iwata used separate powders of barium, yttrium and copper oxides or carbonates, suspended these in aqueous or preferably alcoholic liquid, and then deposited them using direct current voltages ranging from 30 to 300 volts. Iwata repeatedly described his process as being akin to that of "cation paint". Alternatively, Iwata reacted the three critical components to make a superconductive powder which was ground and then deposited onto the substrate electrophoretically. The Iwata process is limited to depositing layers of superconducting material that are thicker than the diameter of dispersed and agglomerated particles.
The process of the present invention is very distinct from both the Koura and Iwata patents in that the metals are deposited one ion/atom at a time from solution, and there is no limitation of minimum thickness per layer.
Further distinction of the present invention relates to the high current density that can be obtained with aqueous solvents, considerably higher than the 0.01 to 10 milliamperes/sq cm preferred by Maxfield. Higher current density means that the electroplating can be accomplished more quickly and more economically by the present invention.
Pawar and Pendse have published a paper "Electrodeposition by Dy-Ba-Cu Alloyed Films from Aqueous Bath," in Materials Research Bulletin, vol 26, 1991, pp 641-648, but they give little data on bath composition. They reported rapid decrease of current density within the first minute of plating which is an indication that the deposit is an electrical insulator, not a metallic layer as in the present invention. They report no complexing agent for this work and found that the thickness decreased after the first 20 minutes because of dissolution.
Minoura et al., supra, report on the codeposition of superconductive elements, with some similarities to Pawar. Minoura used a solvent mixture of dimethyl sulfoxide/water in ratio 10/1 by volume and nitrate salts of the desired metals. The low voltage (-1.4 to -2.0 volts) process deposited barium hydroxide and yttrium hydroxide rather than the metallic elements. As in the experiments of Pawar, the current dropped very quickly; this is due to the deposition of nonconducting hydroxides rather than deposition of metals. Minoura did not discuss use of any complexing agents to facilitate reduction of metallic ions to metals as in electroplating.
In contrast, the present invention deposits metallic elements or barium oxide surrounded by metallic elements (as shown in certain of the examples herein) and current remains relatively constant during the time of deposition. Following electroplating of the metallic elements, the process of this invention involves oxidation of these elements to form a perovskite structure.
The present invention involves the use of complexing chemicals to facilitate the electro-reduction of barium and yttrium ions to elements at the cathode. The use of complexes for these two metals has not been previously reported.
Ethylenedinitrilo-tetraacetic acid and its sodium salt are common complexing chemicals used in electroplating. Citric acid and its salts are not common complexing agents and are not common additions to electroplating solutions. These complexing chemicals are used in the present invention for the purpose of improving the ionic reduction potentials of the desired metals, not for the purpose of chelation or increasing solubility in solution.